1. Field of the Invention
This invention relates to a polyelectrolytic fuel cell capable of generating power by causing fuel gas react to oxidizing gas by means of a polyelectrolytic film.
2. Related Art
Polyelectrolytic fuel cells use cation exchange resin films as their electrolyte. This cation exchange resin films contain a proton (hydrogen ion) exchange base in its molecules, and is conductive. Their hydration to a quasi saturation level results in a relative resistance of 20 .OMEGA..multidot.cm or less at normal temperature enabling them to function as an conductive electrolyte.
The saturated water contents of said polyelectrolytic film changes reversibly depending on temperature. In order to prevent the desiccation of said polyelectrolytic film, fuel gas and oxidizing gas are moisturized before they are supplied. Some moisture contained in said fuel gas and oxidizing gas sometimes condenses to form drops in the passage of gas. During the normal operation of the fuel cell, drops as well as water formed by electrochemical reactions on the side of the oxygen electrode are discharged outside. However, when the quantity of water formed on the side of the oxygen electrode exceeds that of water carried away by the oxidizing gas, it remains in the passage of the oxidizing gas in the form of drops and any drops remaining on the oxygen electrode result in so-called "flooding."
This flooding is a state of drops remaining on the surface (in particular a dispersion layer) of electrodes obstructing the dispersion of gas onto the surface of electrodes. In case of a light flooding where the water contents are relatively small, proportionately to a decrease in the surface of electrodes caused by water remaining thereon, output voltage decreases. However, it is highly possible that air passing through the passage of oxidizing gas carries away moisture and eliminate any flooding thereby and thus output voltage recovers naturally. When a large amount of drops remain on the surface of electrodes or there develops a flooding every time water remains in the passage of oxidizing gas, the surface of electrodes open for gas is substantially reduced. Thus, flooded cells obstruct the supply of oxidizing gas to the oxygen electrodes causing a sharp drop in their voltage, which in turn reduces the output of the whole cells to an insufficient level.
Furthermore, in an advanced stage of flooding, not only the supply of oxidizing gas to the oxygen electrodes is impeded causing a drop in voltage, a drop in the amount of oxygen reaching the oxygen electrodes causes protons (H.sup.+) having passed through a polyelectrolytic film not to react with oxygen on the side of the oxygen electrode and to recombine with electrons (e-) to return to its original form of hydrogen gas (H.sub.2). As a result, the mass of hydrogen gas increases rapidly and this hydrogen gas mixes with oxidizing gas flowing through the passage of oxidizing gas. Thus, the concentration of hydrogen gas resulting from this mixture increases to a highly inflammable level leading to a detonation. The gas mixture sometimes comes into contact with a reaction catalysis layer formed on the surface of the oxygen electrode and ignites to burn intensely.
The mechanism of inflammation in this fuel cell is described below by referring to FIGS. 12 through 14. FIG. 12 is a drawing illustrating the mechanism of mixing hydrogen gas on the cathode side, while FIG. 13 is a cross section of a conventional fuel cell in which a plurality of unit cells standing upright are stacked horizontally. FIG. 14 is a process flow chart starting with the generation of hydrogen gas to inflammation.
As shown in FIGS. 12 and 13, a unit cell 1a of a polyelectrolytic fuel cell 1 comprises an anode 3 comprising a reaction catalysis layer 3a and a gas dispersion layer 3b on the left side of an electrolytic film 2 and a cathode 4 comprising a reaction catalysis layer 4a and a gas dispersion layer 4b on the right side of said film 2. On the outside of said anode 3, a carbon current collector 5 concurrently serving as a gas separator is provided, and on the side of this carbon current collector 5 opposite to said gas dispersion layer 3b, a plurality of parallel fuel gas passages 5a are formed and in each fuel gas passage 5a a fuel gas, i.e. hydrogen gas circulates. On the outside of said cathode 4, a carbon current collector 6 concurrently serving as a gas separator is provided, and on the side of this carbon current collector 6 opposite to said gas dispersion layer 4b, a plurality of parallel oxidizing gas passages 6a (in the drawing only the forefront passage 6a is illustrated) are formed.
When this fuel cell 1 is operated, as the oxidizing gas passages 6a on the cathode 4 side are supplied with air containing oxygen (O.sub.2), on the anode 3 side there occurs a reaction of EQU H.sub.2 =2H.sup.+ +2e.sup.-
on the cathode side there occurs a reaction of EQU 1/2O.sub.2 +2H.sup.+ +2e.sup.- =H.sub.2 O
In other words, on the anode 3, hydrogen gas (H.sub.2) flowing in the fuel gas passage 5a forms protons (2H.sup.+) and electrons (2e.sup.-). Protons moves through the electrolytic film 2 which is an ion exchange film towards the cathode 4, and electrons passes through an external circuit (not illustrated) from a carbon current collector 5 on the anode 3 side to move towards a carbon current collector 6 on the cathode side 4.
And on the cathode 4, oxygen contained in the air flowing in the oxidizing gas passage 6a, protons having moved from the anode 3 into the electrolytic film 2 and electrons having moved via external circuits react each other to form water (H.sub.2 O).
Therefore, water W thus formed in the oxidizing gas passage 6a in each unit cell 1a of the layer-built fuel cell flows down under the pull of gravity and is discharged outside together with excess oxidizing gas from the discharge manifold 7 connected with the lower end of the oxidizing gas passage 6a of each unit cell 1a of the layer-built fuel cell.
Since water thus in each unit cell 1a of the layer-built fuel cell flows down the oxidizing gas passages 6a to converge in the discharge manifold 7, on the outlet side of the discharge manifold 7 (right lower side on FIG. 13) water W tend to increase to stagnate near the outlet. And when water W stagnate near the outlet of the discharge manifold 7, in a unit cell 1a in which the lower end of the oxidizing gas passage 6a is kept open for this stagnating water, discharge is not correctly done and this leads to the stagnation of water W in the oxidizing gas passage 6a. As a result the cathode 4 is inundated causing a flooding of the gas dispersion layer 4b and the reaction catalysis layer 4a and obstructing the dispersion of oxygen gas to this reaction catalysis layer 4a.
Therefore, in a unit cell 1a in which water W in the oxidizing gas passage 6a is smoothly discharged, oxygen gas is supplied to the cathode 4 in a sufficient amount. As a result, there occurs a reaction of EQU 1/20 .sub.2 +2H.sup.+ +2e.sup.- =H.sub.2 O
and no hydrogen gas (H.sub.2) is formed (see the upper half of the oxidizing gas passage 6a of FIG. 12)
A continuous application of current in the state which the water W stagnates in the oxidizing gas passage 6a leads to an insufficient supply of oxygen gas to the cathode 4, causing a drop in the voltage obtained from the whole fuel cell 1. At the same time, no reactions forming H.sub.2 O occurs in the unit cell 1a and proton (2H.sup.+) having passed an electrolytic film 2 and electrons (2e.sup.-) are recombined. As a result, hydrogen gas (H.sub.2) is formed in this oxidizing gas passage 6a. (See the lower half of the oxidizing gas passage 6a of FIG. 12).
And hydrogen gas (H.sub.2) resulting from an insufficient supply of oxygen due to inadequate discharge of water W rises in the oxidizing gas passage 6a of each unit cell 1a and gathers in the supply side of the manifold 8 connected in a manner enabling the supply of oxidizing gas to the top of oxidizing gas passage 6a of each unit cell 1a. It gathers particularly in large amount at the outlet side of the discharge manifold 7, near the farthest end downstream of the supply manifold 8 above the unit cells 1a to which the bottom of the oxidizing gas passages 6a are connected (the right top in FIG. 13). As a result, hydrogen gas stagnating in the supply manifold 8 gradually increases and when its concentration exceeds 4%, the reaction catalysis layer 4a of the cathode 4 serves as a source of ignition and hydrogen gas inflames thereby.
Thus, the inflammation of hydrogen gas in the passage of reaction gas in the fuel cell 1 raises the pressure in the passage and variations in the supply of reaction gas and other factors could result in an instability of voltage generated. In polyelectrolytic cells operated under normal temperature in particular, heat and rising pressure resulting from inflammation could cause variations in voltage generated and produce adverse effects on the seals and other parts of unit cells 1a of the fuel cell stack.
Therefore, it is necessary to control any increases in the concentration of hydrogen gas so that it may not burn intensely, and for this reason it is necessary to dissolve flooding water at its initial stage and to contain as much as possible any progress towards a state of massive generation of hydrogen gas.
In a fuel cell in which hydrogen gas is used as a fuel gas as described above, the flooding of oxygen electrodes with water is a cause of increasing concentrations of hydrogen gas in the passages of oxygen. Therefore, in the invention described in Japanese Patent Application Laid Open No. 54-144934 the flow velocity of the reaction gas is intermittently to increase the pressure difference of the reaction gas in the reaction gas passage, and the drops remaining in the reaction gas passage are eliminated. The adoption of such a method enables to prevent the blocking of gas passages with drops, in other words flooding thereof, to prevent any increase in the concentration of hydrogen gas on the side of oxygen electrodes due to the blocking of passages and thus to prevent possible inflammation of hydrogen gas.
However, in the conventional fuel cell mentioned above in which the flow of reaction gas is regularly increased to push out drops in the gas passages, even in cases where this is not required, the flow rate of reaction gas is increased. As a result, as the flow rate of reaction gas increases to create a sufficient difference of pressure for removing drops, the consumption of reaction gas increases, and following variations in the flow rate of reaction gas the output voltage of a fuel cell varies in a complex manner. Furthermore, in case of abrupt increases in the amount of water within a fuel cell, such water cannot be discharged completely and stagnating water in the passage caused a flooding to develop and progress. Thus, it was highly probable that no action could be taken to prevent a sharp increase in the mass of hydrogen gas developed on the oxygen electrode side.
In the polyelectrolytic fuel cell described above, it was necessary to prevent inflammation inside, in other words to prevent oxidizing gas from mixing in the fuel gas passage and to prevent fuel gas from mixing in the oxidizing gas passage, or remove early gas thus mixed in to prevent inflammation.
Therefore, various inventions have been made so far for example to evacuate efficiently water and to prevent its stagnation in order to prevent hydrogen gas from developing and mixing in the oxidizing gas passages of a fuel cell (See, for example, Japanese Patent Application Laid Open No. 5-251097). However, no attempt has been made so far to prevent inflammation resulting from the generation and mixing in of gas in fuel cells.
In addition, Japanese Patent Application Laid Open No. 4-167367 describes fuel cell equipment comprising cell stacks, a reaction gas pipe line designed to supply and evacuate reaction gas thereto and therefrom, a cell housing and an ambient gas system. In this invention, cell stacks are housed in a cell housing filled with ambient gas, a reaction gas pipe line is connected to these cell stacks and an ambient gas system is connected to said cell housing. In this fuel cell equipment, however, reaction gas (combustible component) having leaked from said cell stacks and mixed in ambient gas is removed by burning the same in the cell housing before it is discharged with ambient gas. For this reason, the art described in this patent application laid open does not enable to remove for example fuel gas that has mixed in the oxidizing gas passage within a fuel cell.